The invention relates to a method for the production of primary aluminium from alumina ore and to an apparatus for performing the method.
The most commonly used method for the production of liquid aluminium from alumina ore is the Hall-Heroult process in which, through electrolysis alumina is decomposed. In this process alumina is continuously fed and dissolved in a bath comprising liquid aluminium and molten cryolite, a mineral comprising fluorides of sodium, aluminium and calcium. A carbon anode is supplied in the bath of molten cryolite and aluminium. The cell containing the bath, is internally coated with lining of a conductive layer facing the inside and acting as a cathode. Liquid aluminium is formed at the cathode and collected on the bottom of the cell. From there it is periodically removed.
The Hall-Heroult process has a number of drawbacks. One drawback is the high, electrical, energy consumption. Another drawback is the emission of fluorides such as CF4 and C2F6 which are considered notorious green house gases, the emission of CO and of heavy metals. Also, the used lining of the cell, known as spent pot lining, is an environmentally unfriendly residu of the Hall-Heroult process. The Hall-Heroult process requires, for a practical production capacity, a large number of cells which together occupy a large area.
Already more than one hundred years ago an alternative process using aluminiumsulfide has been investigated. The investigation has been taken up again in the 1980s, however without success and the process was not pursued further at the time.
U.S. Pat. No. 4,265,716 discloses an embodiment of the aluminiumsulfide based process. In this document it is proposed to react, at a temperature between 1300 K and 1500 K alumina, carbon and sulfur-containing gas to form aluminiumsulfide and carbonmonoxyde. The sulfur containing gas may include sulfur in one or more of several forms, including S2, S6 and CS2. Further it is mentioned that when the gas includes carbon as in CS2, it can replace at least a portion of the carbon otherwise introduced as coke or other solids. U.S. Pat. No. 4,265,716 aims at obtaining molten aluminiumsulfide. The molten aluminiumsulfide is then heated to a temperature of about 1600 K to 1800 K and held there for a sufficient time, about 15 to 60 minutes to cause decomposition to molten AlS and sulfur gas. The molten AlS is then cooled to a temperature sufficient to cause its disproportionation to aluminium and motlen aluminium. This disproportionation is performed in the temperature range of 1200-1370 K.
Although the invention U.S. Pat. No. 4,265,716 was published in 1981, the disclosed process has not been put into practice despite the potential advantages of the aluminiumsulfide process. Obviously, in the field of the production of aluminium from alumina, it had turned out that the proposed process was not feasible. Indeed, tester performed by the present inventors have confirmed this. Surprisingly, it was found that the aluminiumsulfide process wherein use is made of Al2S3 can be performed in a new and inventive manner that is practicable and with which all the advantages as pursued in U.S. Pat. No. 4,265,716 are obtained. Besides, additional advantages over U.S. Pat. No. 4,265,716 are obtained.
These advantages are obtained with a method wherein the conversion of alumina into aluminiumsulfide is performed by reacting alumina with CS2 containing gas at a temperature T2l whereby the alumina is mainly xcex3-alumina like the current alumina used for the Hall Heroult process.
It was found that, different from what was proposed in the prior art the reaction rate of xcex3-Al2O3 and CS2 is very high compared to the reaction rate between xcex1-Al2O3 and CS2. Therefore, the method of our invention has the possibility to yield practical quantities of Al2S3, within a practical time frame. The method of the invention is based on the following net reaction:
2Al2O3+6CS2xe2x86x922Al2S3+6CO+3S2 xe2x80x83xe2x80x83(1) 
COS can be formed as a by-product.
Tests have shown that the transformation from xcex3-Al2O3 to xcex1-Al2O3 takes place in a rather wide temperature range. Therefore, the reaction temperature at which Al2O3 is converted into Al2S3 can be chosen within a wide range depending on other parameters.
In practice it is preferred that the temperature Tal is lower than 1100xc2x0 C., preferably lower than 1025xc2x0 C. more preferably lower than 1000xc2x0 C.
At temperatures above 1100xc2x0 C. all Al2O3 transforms quickly into xcex1-Al2O3. The reaction rate of xcex1-Al2O3 into Al2S3 by means of CS2 proved to be very slow in our experience. Therefore, it is preferred to perform the method according to the invention at a temperature Tal lower than 1025xc2x0 C., more preferably lower than 1000xc2x0 C. In particular above about 1000xc2x0 C. the transformation from xcex3-Al2O3 to xcex1-Al2O3progresses very fast. In practice, this means that a substantial portion of the xcex3-Al2O3has transformed to xcex1-Al2O3 before sufficient Al2S3 has been formed. By operating at a temperature below 1000xc2x0 C., a substantial quantity of Al2S3 can be formed before an imparing quantity of xcex1-Al2O3 has developed. The conversion of Al2O3 to Al2S3 by means of reaction with CS2 is also called sulfidation.
Because the method of the invention is performed at substantial lower temperatures than known in the prior art, a considerable reduction in energy consumption can be achieved. Furthermore, in the method of our invention solid Al2S3 is formed, whereas in the prior art method molten Al2S3 is formed. Therefore, also our method consumes less energy since the melting heat of the formed Al2S3 is saved.
Preferably the temperature Tal is higher than 700xc2x0 C., preferably higher than 750xc2x0 C. Under 700xc2x0 C. the reaction rate is too low for industrial application.
It is preferred that the conversion of alumina into aluminiumsulfide is performed at a conversion pressure higher than 1 Bar absolute pressure.
Tests have shown that the reaction according to equation (1) progresses faster when it is performed at a conversion pressure higher than 1 Bar, which equals about 1 atmosphere. The reaction rate can further be increased with a further embodiment of the invention which is characterised in that the conversion pressure is higher than 5 Bar, preferably higher than 15 Bar. It has shown that, within practical limits, the reaction rate increases with increasing conversion pressure. Therefore, the conversion pressure is selected at a practical optimum, taking into account such parameters as availibility and costs of construction materials for reaction vessels, yield per unit of time and costs and efforts of safety measures.
As mentioned before in the method of our invention, solid Al2S3 is formed. In practice, it may show that the shape of the particles of the bulk Al2S3 is not very suitable for further processing. Therefore, another embodiment of the method of the invention is characterised in that the aluminiumsulfide is at least partly heated to a temperature over its melting temperature. Although the advantage of less energy consumption is reduced, this embodiment has the advantage that Al2S3 is available in a reproducible form, suitable for further processing as will be discussed later.
In the event that molten Al2S3 is not needed or wanted at that stage, a further embodiment of the invention is characterised in that the molten aluminiumsulfide is cooled so as to form small-sized grains, having average grain size smaller than the average grain size of the aluminiumsulfide prior to being heated. This embodiment yields Al2S3 in a form that is easy to handle. Further, this embodiment does not reduce the advantage of the energy consumption essentially since solidification heat can easily be regained. In one possible embodiment the Al2S3 particles formed in the sulfidation are slightly raised in temperature whereby the outer surface of the Al2S3 particles melt. The surface tension causes small-sized densified Al2S3 droplets or particles to develop. Subsequently, these densified Al2S3 droplets or particles are conveyed to a colder part of the reactor e.g. the zone where the sulfidation is taking place, in which the sulfidation takes place, and solidify there. Such densified Al2S3 particles are easy to handle for further processing.
In the sulfidation process CS2 is used as a reactant. Preferably the CS2 is formed from sulfur and a carbonaceous reactant. As carbonaceous reactant, coal, coke, waste materials from the petrochemical industry or waste plastics can be used.
In a preferred embodiment the carbonaceous reactant comprises, preferably contains mainly, methane or natural gas. Methane, in particular in the form of natural gas is available in large quantities and it has the advantage that the production of CS2 can take place with both reactants CH4 and S2 in the gas phase.
Carbondisulfide (CS2) is preferably produced from natural gas and sulfur gas according to the following reaction
CH4+2S2xe2x86x92CS2+2H2 Sxe2x80x83xe2x80x83(2) 
This gas phase reaction is carried out in the temperature window 550-650xc2x0 C. and reaches a conversion of 100%. The reaction is endothermic at these temperature levels and theoretically consumes 1950 kj per kg CS2 when reactants are at 25xc2x0 C. and products at 750xc2x0 C. Most of the heat input goes into dissociation of sulfur vapor to the reactive species S2. In practice 3000 kj per kg CS2 are needed.
World-wide production quantity is about 1.100.000 tonnes a year of which 60% is used in the viscose and rayon industry, and 25% for the production of cellophane and carbontetrachloride. Production volume of CS2 is dropping because cellophane is replaced by other plastic films, carbontetrachloride usage also dropped dramatically because its use as refrigerant and aerosol propellant is driven back. CS2 mixed with air is an explosive mixture over a wide range of concentrations. Together with the low ignition temperature closed installations working above atmospheric pressure to eliminate leaking in of air (oxygen) are used mostly. All equipment containing CS2 must be located well away from potential sources of ignition such as open flames, frictional heat, sparks, electrical light bulbs and bare steam pipes. In practice however only installation working with liquid CS2 have to be protected in such a way. Leakage from hot installation parts will not result in dangerous CS2 clouds but in small flames where CS2 reacts with oxygen to CO2 and SO2, thus eliminating danger of explosion.
In the production of CS2 in accordance with equation (2) also H2S is produced. A preferred embodiment of the method of the invention is characterised in that hydrogensulfide (H2S) formed in the production of CS2 is removed and converted to form sulfur which sulfur is returned for the production of CS2. The produced H2S can be subjected to the following reaction:
3H2S+1,5O2xe2x86x923S÷3H2Oxe2x80x83xe2x80x83(3) 
The sulfur can be re-used for the production of CS2. In this manner the supply of make-up sulfur can be reduced.
Another preferred embodiment is characterized in that unreacted sulfur in the production of CS2 is removed, preferably by condensation, and returned for the production of CS2. In this way, the CS2 is purified and sulfur is re-used and less sulfur needs to be supplied from external sources.
Yet another embodiment of the method of the invention is characterised in that the CS2 used is formed essentially from sulfur of which the mainstream result from the separation of the aluminiumsulfide into aluminium and/or sulfur from the conversion of alumina into aluminiumsulfide. In this embodiment, practically all sulfur used in the conversion of Al2O3 into Al and by-products, is re-used and only small amounts that inevitably are lost, need to be supplied from external sources.
The invention is also embodied in a method in which prior to the conversion of alumina into aluminiumsulfide (sulfidation) the alumina is dried and pressurised, whereupon the sulfidation is performed by passing a gas-solid mixture containing gaseous CS2 and solid alumina through a reactor at a temperature of preferably between 800xc2x0 C. and 900xc2x0 C. and at a pressure of preferably between 5 and 35 Bar, whereupon the solids are separated and the gas is further treated for separating unreacted CS2 and by-products such as CO, COS and S2, at least one of which is fed back into the process for the production of CS2. In this method the sulfidation is performed in a preferred temperature and pressure range, taking into consideration constructional parameters, energy consumption and unavoidable side reactions. By-products are to a larger extent re-used in the process.
An embodiment of the invention that is particularly advantageous, is characterised in that the CS2 containing gas is formed and essentially fed directly, without essential intermediate storage, to a reactor vessel to react with alumina to form aluminiumsulfide. According to this embodiment it is proposed to integrate the production of Al2S3 with the sulfidation and not to acquire CS2 from remote production facilities.
Integrating CS2 production with aluminium production in particular with the sulfidation therein has the following advantages:
No provisions have to be made to store and distribute liquid CS2. Only a CS2 storage tank is needed for start-up.
Further no provision have to be made to receive and store large quantities of liquid sulfur while the sulfur in this process can be recycled for nearly completely in accordance with other preferred embodiments.
The final step in the CS2 process is normally a destillation to remove H2S from the liquid CS2 to obtain a 99.9% pure liquid CS2. This step can be optionally omitted while H2S will have no negative effect on the sulfidation process.
A totally different reactor design can be applied for the production of CS2 from methane or natural gas and sulfur. Generation of hot sulfur gas in the electrolysis of Al2S3 eliminates the need to vaporize sulfur in the CS2 reactor. In the new reactor design the temperature may be chosen higher, making the reaction of methane or natural gas and sulfur exothermic instead of endothermic. This will have the additional advantage in eliminating the use of methane or natural gas as a fuel in the CS2-reactor.
The off-gas of the sulfidation reactors contains unreacted CS2, S2 and CO. This gas can be cleaned in the gas cleaning section of the CS2 plant. The CO can eventually be fed into the combustion chamber of a Claus unit for the production of sulfur, where it is burned to CO2 and will attribute to the production of super heated high pressure steam in the Claus waste heat exchanger.
Preferably the CS2 containing gas is essentially CS2. It is not necessary, for the sulfidation, to use essentially CS2 as CS2 containing gas. However, to avoid possible side reactions and to save energy it is preferred to use essentially CS2 as CS2-containing gas.
From the Al2S3, produced in the sulfidation, metallic aluminium is to be made. In a preferred embodiment the separation of aluminium from aluminiumsulfide is performed by electrolysis.
As discussed above, the Hall-Heroult process, in which molten metallic aluminium is made through electrolysis, has many drawbacks.
In the 1980""s experiments were conducted directed to the production of aluminium in a chloride process in which aluminiumchloride was produced and subsequently subjected to electrolysis. The chloride process was abandoned in 1985. One of the main reasons being the inevitable production of environmentally hazardous chlorinated hydrocarbons during the production of aluminiumchloride. This would lead the skilled person away from applying electrolysis for the separation of Al from Al2S3.
However the inventors have realised that the electrolysis process as developed for the aluminiumchloride-based process can advantageously be further developed for application in the aluminiumsulfide-based process. The electrolysis of aluminiumsulfide is a further development of the electrolysis of aluminiumchloride. Similar advantages over the Hall-Heroult process exist, whereas the disadvantages can be less because of the less aggressive nature of sulfur containing gases in comparison to chloride containing gases. Also spent materials like refractory can be treated more easily to obtain environmentally safe waste products. The spent pot lining from the present process will only contain sulfur and chlorides, but will be essentially free from fluorides and cyanides.
Also the working conditions around the electrolysis cell will be better because the electrolysis cells should be closed to prevent air leaking in and causing oxidation of the Al2S3.
A further embodiment of the method of the invention is characterised in that the electrolysis is performed in a multi-polar electrolysis cell.
The multi-polar cell has the advantage that the voltage drop in the electrolysis process can be reduced due to a low resistance of the cell. Such cell is known e.g. from U.S. Pat. No. 4,133,727.
Still a further embodiment of the method of the invention is characterised in that the electrolysis is performed directly in a bath of molten aluminiumsulfide.
As an alternative liquid Al2S3 can be electrolysed directly (Al2S3 being the most abundant single component in the melt) with only very small amounts and preferably no external addition of salts to the melt, with or without the use of membranes. The most important advantage of this embodiment is that small inter-electrode space is possible (no lack of feedstock between the electrodes).
The invention is also embodied in an apparatus comprising a first reactor for the manufacture of CS2, a second reactor for the manufacture of Al2S3 from CS2 and Al2O3 and a third reactor for the manufacture of Al from Al2S3 said third reactor preferably being an electrolysis cell. Such apparatus has the advantage that the CS2 production is integrated with other process steps for the production of aluminium from alumina and that intermediate products need to be conveyed over short distances. This is of particular advantage where intermediate products are aggressive or at high temperature.
The process of the invention is also referred to as the Compact Aluminium Production Process or CAPP, the key feature of this process, being the conversion of aluminiumoxide (alumina) to aluminiumsulfide, which can be converted into sulfur (gas) and aluminium preferably through electrolysis.